The Earth appears to have been warmer since 1980 than at any time in the last 18 centuries, scientists say.

The climate sceptics are flogging a dead horse

Professor Philip Jones, University of East Anglia

They reconstructed the global climate from data derived from ice cores, vegetation and other records.

They believe their research provides unequivocal confirmation that humans are affecting the climate.

But sceptics still insist that any human contribution is likely to be too small to explain what is happening.

The scientists are Professor Philip Jones, of the climatic research unit, University of East Anglia, UK, and Professor Michael Mann, of the University of Virginia, US.

Their study, published in the journal Geophysical Research Letters, supports recent findings from the Intergovernmental Panel on Climate Change.

After studying temperature data from up to 1,000 years ago, the panel said the late 20th century had been the warmest period on record.

To test the strength of claims that the world had in fact been warmer before 1000 AD, Professors Jones and Mann sought to reconstruct the global climate over the last two millennia.

Frozen evidence

They examined the trunks of ancient trees from different regions to compile a record of local conditions - the thickness of the trees' annual growth rings is determined by the climate.

More grapes grow in UK now

They also studied cores drilled from the icecaps of Greenland and Antarctica, examining the trapped air bubbles for information about the climate prevailing at the time the ice formed.

A third source of information was historical records, especially from the Netherlands, Switzerland and China.

The authors were unable to find enough information to work out what the southern hemisphere's climate had been, but are satisfied their conclusion that the northern half of the planet is in the warmest period of the last 2,000 years is robust.

While some parts of the Earth may have been warmer than they are now, they say, average temperatures were cooler.

They say the Earth has warmed by at least 0.2C in the last 20 years or so - the amount by which it has warmed or cooled over the space of a century in the past.

Fingering the Sun

Professor Jones said: "It just shows how dramatic the warming has been in recent years. You can't explain it in any other way - it's a response to a build-up of greenhouse gases in the atmosphere."

The Vikings cared little for climate

Some scientists believe the recent temperature increases are explained by solar radiation, with emissions of carbon dioxide and other greenhouse gases too small to account for the changes observed.

To the argument that northern Britain was warm enough 1,000 years ago for vineyards to flourish, the authors say there are far more now.

Seeing the whole

They say the Vikings' voyage from Iceland to Greenland in 980 AD was a quest for land, not for a warmer climate.

They also reinterpret the fact that the river Thames used to freeze over more often, saying the design of the original London Bridge affected the river and made it freeze more easily.

Professor Jones told BBC News Online: "The climate sceptics are flogging a dead horse. You can't say the whole world was once warmer than it is now just because Europe was warmer.

"You have to aggregate the records together, as we've done. We'd like more records, especially from the tropics - but we do think we have enough information to say the world is now warmer than it's been for 2,000 years."